Recent research in this laboratory has demonstrated that 4-month-old infants recognize the correspondence between auditorially and visually presented speech sounds. They recognize that particular sounds emanate from mouths moving in particular ways, thus demonstrating one of the components of "lip-reading" in adults. This ability has important implications for the development of sensory and perceptual processing, particularly of speech, in normal infants. The experiments proposed here extend our research on auditory-visual speech perception in infants in four ways. First, the research examines the extent of the effect, extending the experiments to more difficult examples. Second, it examines the development of the effect, extending the age range tested to under three weeks of age. Third, the proposal examines the basis of the effect, testing whether infants recognize auditory-visual correspondences when complex nonspeech sounds are used. Fourth, it examines the nature of the interaction in auditory-visual speech perception, using experiments that probe the nature of the metric by which optic and acoustic information for speech is equated. At a theoretical level, the experimental outcomes are directly relevant to models of speech perception and vocal learning in infancy and should also enrich our understanding of social and cognitive development in normal infants. The data may also impact our understanding and treatment strategies for infants born deaf or blind.